Copper foil for printed circuits

ABSTRACT

There is provided a copper foil for printed circuits characterized by having a coating layer which contains zinc and zinc oxide, chromium oxide and nickel, and either or both of zinc and zinc oxide at least on the shiny side of a copper foil. The coating layer is formed by electroplating using a plating solution which contains either or both of zinc salt and zinc oxide, chromic salt and nickel salt. This copper foil resists discoloration upon exposure to high-temperature conditions of 180° C. for 30 minutes. Resist adhesion is also good. The amounts of deposits are preferably 60 to 80 μg Zn, 30 to 40 μg Cr and 5 to 20 μg Ni per dm 2 . The matt side of the copper foil may be treated to form thereon a single metal layer or alloy layer of one or more metals chosen from among Cu, Cr, Ni, Fe, Co, and Zn. The copper foil is suited for the fabrication of printed circuits with fine circuit patterns.

FIELD OF THE INVENTION

This invention relates to a copper foil for printed circuits and aprocess for producing the same, and more particularly to a copper foilfor printed circuits with improved resist adhesion and good thermaloxidation resistance on the shiny side thereof and also to a process forproducing the same.

BACKGROUND OF THE INVENTION

A copper foil for printed circuits generally is laminated and bonded toa base of a synthetic resin or the like at a high temperature and a highpressure, screen-printed with a necessary circuit pattern using a resistto form an objective circuit, and then is etched with an etchant such ashydrochloric acid to remove unnecessary portions. Finally, necessaryelements are soldered in place, and in this way various printed circuitboards for electronic devices are fabricated. Qualitative requirementsfor the copper foil for printed circuit boards differ with the sidesurfaces, the surface of the side to be bonded to the resin base (mattside) and the surface of the side not to be bonded to the resin base(shiny side).

Requirements for the matt side chiefly include:

(1) No possibility of oxidative discoloration during storage (corrosionprotection, rust prevention);

(2) Adequate resistance to peeling from the base even afterhigh-temperature heating, wet treatment, soldering, chemical treatmentor the like (peel strength); and

(3) Freedom from so-called lamination spots that can be observed afterlamination to the base and etching (hydrochloric acid resistance).

Requirements for the shiny side include:

(1) Good appearance and no oxidative discoloration during storage(corrosion protection, rust prevention);

(2) Good surface propensity of being wetted by solder (solderwettability);

(3) No oxidative discoloration upon high-temperature heating (thermaloxidation resistance); and

(4) Good adhesion to resist (resist adhesion).

To meet these requirements, varied processes for diverse purposes havehitherto been proposed for the treatment of copper foil for printedcircuit boards, respectively of its matt and shiny sides. Payingattention to the protection of copper foil against oxidation orcorrosion in particular, the present applicant previously proposed, asJapanese Patent Application Publication No. 33908/1986, a processcomprising forming a zinc coating film on the shiny side of a copperfoil and then forming a chromium oxidation preventive coating layer of achromium oxide film. The same applicant also proposed and achieved manyfavorable results from the formation of a mixed coating film of zincand/or zinc oxide and chromium oxide by electrolytic zinc-chromiumtreatment (Japanese Patent Application Publication No. 7077/1983).Furthermore, Patent Application Public Disclosure No. 294490/1990disclosed another process which consists in forming a chromium oxidefilm by immersion chromate treatment and then forming a mixed coatingfilm of zinc and/or zinc oxide and chromium oxide by said electrolyticzinc-chromium treatment for the purpose of preventing black spotting oncopper foil which may be caused when it is exposed to hot, humidconditions for a long time period.

Recently, of among the properties required of the shiny side of copperfoil, adhesion to resists, such as plating resist and etching resist,has become particularly important along with thermal oxidationresistance. Good resist adhesion to copper foil is a key to enhancedetching precision to keep up with the tendency toward finer patterns forprinted circuits. If resist adhesion is inadequate, infiltration of theetchant such as hydrochloric acid into the region under the resist filmwould render it difficult to form fine circuit patterns having a verynarrow width.

In the procedure of resist printing, it has been customary tomechanically polish the copper foil prior to printing so as to ensuregood adhesion of resist to the copper foil.

More recently, for the saving of labor for the process and with thethinning of copper foil for higher density printed circuit boards thanbefore, the trend is toward the omission of the mechanical polishing forenhanced resist adhesion. On the other hand, as noted above, growingimportance is attached to the resist adhesion in view of the finercircuit patterns of recent and future printed circuits. Anycountermeasure therefore is required which would improve the resistadhesion to thin copper foils without an increase in the number ofprocess steps needed. Needless to say, the countermeasure should notimpair not only the other properties the shiny side must have, such asthermal oxidation resistance and solder wettability, but also the manyrequisites of the matt side, including peel strength and resistance tohydrochloric acid attack.

OBJECT OF THE INVENTION

The object of the present invention is to establish a technology ofimproving the resist adhesion to the shiny side of copper foil forprinted circuits without impairing the other properties on the shinyside such as corrosion protection ability, thermal oxidation resistance,and solder wettability, or the other properties required of the mattside such as peel strength and hydrochloric acid resistance, and withoutthe need of increasing the number of process steps involved.

SUMMARY OF THE INVENTION

The present inventors have made intensive research on the basis of theirjudgment that, in view of the excellent properties of the conventionalmixed coating film of zinc and/or zinc oxide and chromium oxide, itwould be the best approach to improve the resist adhesion while makingmost of excellent properties of this mixed coating film. As a result, ithas now been found that the foregoing object can be attained byincluding nickel into the mixed coating film of zinc and/or zinc oxideand chromium oxide.

Based on the above discovery, the present invention provides, in its oneaspect, a copper foil for printed circuits characterized by having acoating layer which contains either or both of zinc and zinc oxide,chromium oxide and nickel at least on a shiny side of a copper foil.Preferably, in the coating layer, the amount of Zn deposited is in therange of 60 to 80 μg/dm², the amount of Cr deposited is in the range of30 to 40 μg/dm² and the amount of Ni deposited is in the range of 5 to20 μg/dm².

In a preferred embodiment, the present invention provides a copper foilfor printed circuits characterized by having at its matt side a treatedlayer of a single metal layer or alloy layer of one or two or moremetals selected from the group consisting of Cu, Cr, Ni, Fe, Co and Znand a coating layer formed on said treated layer and containing eitheror both of zinc and zinc oxide, chromium oxide and nickel and on theother hand having at its shiny side a coating layer containing either orboth of zinc and zinc oxide, chromium oxide and nickel.

In another aspect, present invention provides a process for imparting toat least the shiny side of a copper foil good resist adherence andthermal oxidation resistance for printed circuits characterized byelectroplating at least the shiny side of the copper foil with a platingsolution which contains either or both of zinc salt and zinc oxide,chromic salt and nickel salt to form a coating layer which containseither or both of zinc and zinc oxide, chromium oxide and nickel atleast on the shiny side of the copper foil.

The present invention further provides a process for producing a copperfoil for printed circuits characterized by electroplating at least theshiny side of a copper foil with a plating solution which containseither or both of zinc salt and zinc oxide, chromic salt and nickel saltto form a coating layer which contains either or both of zinc salt andzinc oxide, chromium oxide and nickel at least on the shiny side of thecopper foil.

Electroplating is preferably conducted in the following plating solutioncomposition and operation conditions.

CrO₃ : 0.5-10 g/l

Zn: 0.1-10 g/l

Ni: 0.01-10 g/l

Na₂ SO₄ : 5-20 g/l

pH: 3.0-5.5

Temperature: 30-55° C.

Current density: 0.5-4.0 A/dm₂

Plating time: 1-5 sec.

Also in the conventional mixed coating film of zinc and/or zinc oxideand chromium oxide, resist adhesion can be improved by adjusting theamounts of zinc and chromium deposits. However, this involves asacrifice of thermal oxidation resistance, and simultaneous attainmentof the both properties has been impossible.

The coating layer according to the invention consists of zinc and/orzinc oxide (namely, either or both of zinc and zinc oxide), chromiumoxide and nickel. The addition of nickel to the conventional combinationof zinc and/or zinc oxide and chromium oxide improves the resistadhesion to the shiny side of a copper foil while maintaining thecorrosion protection ability, thermal oxidation resistance and solderwettability of the shiny side and the peel strength, hydrochloric acidresistance and other properties of the matt side on necessary levels.Nickel enhances the resist adhesion of the shiny side without adverselyaffecting the outstandingly advantageous properties of the combinationof zinc and/or zinc oxide and chromium oxide. Moreover, this coatinglayer can be formed in a single step of plating.

EXPLANATION OF EMBODIMENT OF THE INVENTION

The copper foil to be used in this invention is either electrolyticcopper foil or rolled copper foil. One side surface of the copper foilis roughened and preferably that roughened side is treated so as to formthereon a single metal layer or alloy layer of one or two or more ofmetals selected from the group consisting of Cu, Cr, Ni, Fe, Co and Zn.When occasion demands, the roughening is preceded by or followed byordinary copper plating as a pretreatment or finishing treatment.

The roughening as stated above is done usually on the surface of acopper foil to be bonded to a resin base, i.e., on the surface to beroughened and laminated to the base (matt side), for the purpose ofincreasing the peel strength of the foil after lamination with respectto the resin base. For example, the surface of the copper foil to beroughened is degreased and then subjected to copper roughening treatmentby which knurly or grainy copper electrodeposits are formed on thesurface. Such grainy copper electrodeposits can be easily formed byso-called burnt electrodeposition. In a typical copper rougheningtreatment, e.g., the following conditions may be adopted:

Copper roughening treatment

Cu: 10-25 g/l

H₂ SO₄ : 20-100 g/l

Temperature: 20-40° C.

D_(k) : 30-70 A/dm²

Time: 1-5 sec.

As stated above, following the roughening treatment, it is desirablethat the matt side be treated to form thereon a single metal layer oralloy layer of one or two or more metals chosen from among Cu, Cr, Ni,Fe, Co and Zn. Examples of alloy plating include Cu-Ni, Cu-Co, Cu-Ni-Coand Cu-Zn. (For details, refer to Japanese Patent ApplicationPublication No. 9028/1981, Japanese Patent Application Public DisclosureNos. 13971/1979, 292895/1990, and 292894/1990, and Japanese PatentApplication Publication Nos. 35711/1976 and 6701/1979.) This treatmentserves to dictate the final properties of the copper foil and alsoprovides a barrier for the copper foil. For an information, anelectrolyte composition and operation conditions for Cu-Zn treatmentwill be given below:

Cu-Zn treatment

NaCN: 10-30 g/l

NaOh: 40-100 g/l

CuCN: 60-120 g/l

Zn(CN)₂ : 1-10 g/l

pH: 10-13 g/l

Temperature: 60-80° C.

D_(k) : 1-10 A/dm²

According to this invention, a coating layer containing zinc and/or zincoxide, chromium oxide, and nickel is formed on at least the shiny sideof a copper foil, with or without the aforementioned concomitanttreatment of the matt side thereof.

The coating treatment of the invention (hereinafter called a"zinc-chromium-nickel treatment") is performed in a single plating tankby electroplating using a plating solution containing at least onechromic salt chosen from among K₂ Cr₂ O₇, Na₂ Cr₂ O₇, and CrO₃, a zincsalt such as ZnSO₄. 7H₂ O and/or ZnO, and a nickel salt such as NiSO₄.The process involves no extra steps compared with conventionaltreatments. For added electric conductivity of the treating solution,the addition of Na₂ SO₄ is advisable. The composition of the treatingsolution and the conditions used under this invention are as follows:

zinc-chromium-nickel treatment

CrO₃ : 0.5-10 g/l

Zn: 0.01-10 g/l

Ni: 0.01-10 g/l

Na₂ SO₄ : 5-20 g/l

pH: 3.0-5.5

Temperature: 30-55° C.

Current density: 0.5-4.0 A/dm₂

Plating time: 1-5 sec.

The conditions for electrolysis to form such a mixed coating film areextremely delicate. The composition of the treating solution, bathtemperature, current density, plating time, and other parametersinterrelatedly influence the properties of the resulting coating filmand so it is difficult to readily define an absolute combination ofthese conditions satisfying all requirements. The electrolyticconditions given above are stated as individually feasible ranges. Ofthe ranges specified, a combination of values best suited to meetparticular requirements must be chosen. In brief, it is only necessaryto form a coating film of uniform thickness that contains zinc and/orzinc oxide, chromium oxide and nickel and is fully densified and isclosely adherent to a copper foil.

In the zinc-chromium-nickel treatment according to the invention, it isdesirable to deposit Zn, Cr and Ni in amounts within the ranges of 60 to80 μg/dm², 30 to 40 μg/dm² and 5 to 20 μg/dm², respectively, whereby theresist adhesion to the shiny side can be improved while maintaining thecorrosion protection ability, thermal oxidation resistance and solderwettability of the shiny side and the peel strength, hydrochloric acidresistance and other properties of the matt side on favorable levels.The combination of the deposited amounts of Zn and Cr in the specificranges is effective for the attainment of the above-mentionedproperties. If the amount of Ni deposited is less than 5 μg/dm², itseffect upon the improvement of resist adhesion is not adequate.Conversely, if the Ni amount exceeds 20μg/dm², it tends to sacrifice thehighly desirable properties of the mixture of zinc and/or zinc oxide andchromium oxide. The thickness of the coating layer may differ with thematt and shiny sides, depending on the property requirements for therespective sides.

Lastly, when necessary, principally for the purpose of improving theadhesion between the copper foil and the resin base, silane treatment isconducted whereby a silane coupling agent is applied to at least thematt side of the corrosion protection coating layer. The method ofapplication of a silane coupling agent solution may be by spraying,application by a coater, immersion or flow coating. For instance, PatentApplication Publication No. 15654/1985 discloses that the adhesionbetween a copper foil and a resin base is increased by chromate-treatingthe matt side of the copper foil followed by treating with a silanecoupling agent. For the details, refer to this particular publication.

When the necessity arises thereafter, the copper foil may be annealedfor improving its ductility.

The copper foil thus obtained exhibits good resist adhesion andsufficient thermal oxidation resistance to remain discolored under thehigh temperature conditions of 180° C. for 30 minutes. Conventionalprocesses for zinc-chromium treatment have had to sacrifice thermaloxidation resistance in order to obtain good resist adhesion andcontrariwise have had to sacrifice the resist adhesion in order toobtain thermal oxidation resistance of the level of about 180° C. forabout 30 minutes. This invention, on the other hand, provides a copperfoil which simultaneously possesses both of these desirable propertiesof resist adhesion and thermal oxidation resistance. The copper foilpossesses good solder wettability too. Thus the copper foil of theinvention, in the process of fabricating printed circuits, shows goodadhesion to resists, such as plating resist and etching resist andtherefor the infiltration of the treating solution from between theresist and the base through the edges is negligible. Consequently, finecircuits having narrower width can be made with high precision. Thecopper foil, in addition, can withstand the thermal hysteresis it willexperience during the fabrication of printed circuit boards.

EXAMPLES

An electrolytic copper foil was (1) roughened, the roughened side (mattside) was (2) Cu-Zn-treated, and both the matt and shiny sides were (3)zinc-chromium-nickel-treated. The conditions used for these threetreatments were as follows:

(1) Copper roughening

Cu: 15 g/l

H₂ SO₄ : 50 g/l

Temperature: 30° C.

D_(k) : 50 A/dm²

Time: 3 sec.

(2) Cr-Zn treatment

NaCN: 20 g/l

NaOH: 60 g/l

CuCN: 80g/l

Zn(CN)₂ : 5 g/l

pH: 12

Temperature: 70° C.

D_(k) : 5 A/dm²

(3) Zinc-chromium-nickel treatment

CrO₃ : 1.3 or 2.5 g/l

Zn: 0.5 g/l

Ni: 0 or 0.1 g/l

Na₂ SO₄ : 10 g/l

pH: 5.0

Temperature: 50° C.

Current density: 0.5-2.5 A/dm²

Plating time: 1.5-3.6 sec.

The products thus obtained were tested, on the shiny side, to determinethe deposition amount of surface film, resistance to baking test, solderwettability, and resist adhesion. For the surface analysis, after thematt side was masked by pressing with a base material, such as onemarketed under the trade designation "FR-4", the masked foil wasimmersed in an acid to dissolve Zn, Ni, and Cr out only from the shinyside which were analyzed by atomic absorption spectroscopy. For thebaking test, test pieces were placed into an oven of a preselected type,taken out, and inspected to see if the shiny side had discolored(criteria: ∘=not discolored; X=discolored). As for the solderwettability, a pressed copper foil laminated base was dipped verticallyinto a soldering tank using as the flux a product of San-ei Chemical Co.marketed under the trade name "JS64". The wetting angle of the solderdrawn up along the laminate surface was measured. The smaller the anglethe better the solderability. Resist adhesion was evaluated in thefollowing manner. Each copper foil piece was placed on a prepreg piece,pressed together into a laminate, and the laminate was screen-printedwith a given pattern using an ultraviolet-curing resist ("UR-450B"marketed by Tamura Kaken Co.). The printed laminate was passed at apredetermined speed through an ultraviolet irradiation apparatus and theresist was dried. The finished sample was immersed in 5% HCl at 50° C.for 5 minutes, water rinsed, dried, and the surface was inspected to seeif the resist had peeled off or not (criteria: ∘=not peeled; X=peeled).With regard to the matt side, each test copper foil was securelylaminated on that side to a glass-cloth-based epoxy resin board, and thepeel strength and hydrochloric acid resistance were evaluated in theknown and established manner. The test results are given in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                 Amount                                                                        deposited        Solder wettability                              CrO.sub.3                                                                            Zn Ni (μg/dm.sup.2)                                                                     Baking test                                                                             Wetting                                                                            Wet-                                                                              Resist                                 No.                                                                              (g/l)                                                                             (g/l)                                                                            (g/l)                                                                            Zn Ni                                                                              Cr                                                                              180° C. × 30 min.                                                          angle                                                                              ability                                                                           adhesion                               __________________________________________________________________________    1  2.5 0.5                                                                              -- 64 --                                                                              37                                                                              X         59.8 100 ◯                          2  2.5 0.5                                                                              -- 74 --                                                                              42                                                                              ◯                                                                           62.4 100 X                                      3  1.3 0.5                                                                              -- 68 --                                                                              34                                                                              X         46.1 100 ◯                          4  1.3 0.5                                                                              -- 120                                                                              --                                                                              46                                                                              ◯                                                                           49.2 100 X                                      5  1.3 0.5                                                                              0.1                                                                              71 12                                                                              33                                                                              ◯                                                                           47.1 100 ◯                          6  1.3 0.5                                                                              0.1                                                                              69 10                                                                              37                                                                              ◯                                                                           55.4 100 ◯                          __________________________________________________________________________

The test results of Table 1 show that coating films of Nos. 1 to 4 thatwere conventionally zinc-chromium-treated and free from nickel, or mixedcoating films of zinc and/or zinc oxide and chromium oxide, failed tosatisfy the both requirements of thermal oxidation resistance (in thebaking test at 180° C. for 30 min.) and resist adhesion, even though thezinc and chromium contents in the films were varied. In order to satisfyeither one of thermal oxidation resistance or resist adhesionrequirement, the other had to be sacrificed.

It can also be seen, by contrast, that the films of Nos. 5 and 6 thatwere zinc-chromium-nickel-treated in conformity with this invention, orthe mixed films of zinc and/or zinc oxide, chromium oxide and nickel,were superior in both thermal oxidation resistance and resist adhesionowing to the presence of nickel. Solder wettability was also good at100%. Corrosion protection ability too was favorable.

The results evaluated above pertain to the shiny side. The properties ofthe matt side, such as peel strength and resistance to hydrochloric acidattack, were satisfactory with all the samples of Nos. 1 through 6 andgave substantially the same values as regards those properties.

ADVANTAGES OF THE INVENTION

The copper foil product of the present invention is improved in resistadhesion over the conventional zinc-chromium-treated copper foilproducts and is superior in both thermal oxidation resistance and resistadhesion. It undergoes no discoloration of the shiny side by the thermalhysteresis it experiences during the fabrication of printed circuitboards. The improved resist adhesion makes it suitable for fine circuitfabrications. Further, the coating according to this invention does notimpair the other properties of the shiny side, such as oxidationprotection and solder wettability, and the peel strength, hydrochloricacid resistance, etc. of the matt side. With these advantages it isexpected to meet the future requirements for the copper foil for printedcircuits. In addition, the coating treatment is performed by single-stepplating, and therefore adds practically nothing to any cost in theprocess.

What we claim is:
 1. A copper foil for printed circuits having a shinyside with a coating layer thereon, the coating layer comprising chromiumoxide and nickel, and either or both of zinc and zinc oxide.
 2. Thecopper foil of claim 1, wherein the amount of zinc in the coating layeris in the range of 60 to 80 μg/dm², the amount of chromium in thecoating layer is in the range of 30 to 40 μg/dm², and the amount ofnickel in the coating layer is in the range of 5 to 20 μg/dm².
 3. Acopper foil for printed circuits comprising:(a) a shiny side having acoating layer comprising chromium oxide and nickel, and either or bothof zinc and zinc oxide; and (b) a matt side having a treated layer ofeither a single layer or an alloy layer of one or more metals selectedfrom the group consisting of Cu, Cr, Ni, Fe, Co and Zn, and a coatinglayer formed on the treated layer, the coating layer comprising chromiumoxide and nickel, and either or both of zinc and zinc oxide.
 4. Thecopper foil of claim 3, wherein the amount of zinc in the coating layerof the matt side and shiny side is in the range of 60 to 80 μg/dm², theamount of chromium in the coating layer of the matt side and shiny sideis in the range of 30 to 40 μg/dm², and the amount of nickel in thecoating layer of the matt side and shiny side is in the range of 5 to 20μg/dm².